Earthquakes
The shaking of earth caused by waves moving on and below the earth's surface and causing: surface faulting, tremors vibration, liquefaction, landslides, aftershocks.
The crust of Earth is actually broken into 12 main plates that fit together like puzzle pieces.
The mantle is the layer of Earth just below the crust. It is a made up of a liquid.
Because the mantle is liquid, the hard surface of the Earth is actually floating.
The space between two plates is called a fault.
Because earthquakes occur when two plates rub together, they are often said to occur on fault lines.
Earth’s plates can move in three directions:
Divergent, Convergent & Transformational.
The Transformational movement forms an earthquake.
When plates try to move in opposite directions, they put pressure on each other because they do not want to slide past each other.
When the pressure gets too great the pressure breaks and the plates are forced to slide. This sliding, called friction, is rough and jerky which causes the Earth to shake.
The point where the plates experience the friction is called the epicenter.
The shaking starts at the epicenter and spreads in circles outward much like the ripples of water dripping into a puddle.
Earthquakes which occur on the ocean floor produce giant sea waves called tsunamis. Tsunamis can travel at speeds of 700 to 800 km per hour. As they approach the coast, they can reach heights of greater than 20 meters.
The waves of shaking are called seismic waves.
The Richter Scale:-
The Richter scale is used to measure how much shaking an earthquake causes.
The Richter scale gives each earthquake a number from 1-10.
1). Less than 2 – cannot be felt
2). 2 and 4 – can be felt but do not cause damage
3). 5 and 6 – can cause damage
4). 7-10 are major earthquakes and cause severe damage.
Earthquakes are measured using a seismometer.
Some Important Earthquakes:-
1755 - Lisbon, Portugal (Killed 70,000, Raised Waves in Lakes all over Europe)
First Scientifically Studied Earthquake
1811-1812 - New Madrid, Missouri
Felt over 2/3 of the U.S. (Few Casualties)
1886 - Charleston, South Carolina
Felt All over East Coast, Killed Several Hundred.
First Widely-known U.S. Earthquake
1906 - San Francisco (Killed 500 (later studies, possibly 2,500) )
First Revealed Importance of Faults
1923 – Tokyo - Killed 140,000 in firestorm
1964 - Alaska (Killed about 200 )
Wrecked Anchorage.
Tsunamis on West Coast.
1976 - Tangshan, China
Hit an Urban Area of Ten Million People Killed 650,000
2005 - Pakistan, BALAKOT earthquake death toll rises to 79,000 (October 8, 2005) , Pakistan's death toll soared to 79,000 from South Asia's earthquake after a survey of one of the two hardest-hit Pakistani regions -- making it one of the deadliest quakes in modern times.